Simulated succession under high grazing Spatial Simulation of the Dynamics of Establishment of Secondary Forest in Abandoned Pasture in the Central Amazon Karin T. Rebel*, Susan J. Riha*, Marco A. Rondon*, Ted.R. Feldpausch*, Elisa Wandelli ‡ , Erick C.M. Fernandes* *Cornell University, Ithaca, NY, ‡ Centro de Pesquisa Agroflorestal da Amazônia (EMBRAPA-CPAA), Manaus, AM ND04_FEER_03 Model We developed a spatial, dynamic model to simulate the rate of conversion into secondary forest of Central Amazonian pastures subject to different management systems. We used the software PCRaster, which discretizes the landscape into cells, to simulate the conversion process. In PCRaster, GIS operations or other functions can induce changes in cell attributes, which can be boolean, nominal, ordinal, scalar or directional, on the basis of the relations within cells or between cells. New cell attributes can be computed as a function of attribute change over time. Thus this software allows us to represent both spatial and time dependent processes simultaneously. Introduction During the last two decades, over 50 million hectares of Amazonian forest have been cleared and converted into pastures. Most of the area under pastures experience a rapid decline in grass productivity, which results in their abandonment. Weeds rapidly invade abandoned pastures and become dominant within one to two years. A few years later, woody species begin to dominate abandoned pastures and initiate the re-conversion of the land to forest. The rate of conversion and accumulation of biomass is considered to be dependent on the history of land use. Management practices such as the frequency of burning and grazing intensity, among others, could influence the rate of succession and biomass accumulation. The purpose of this study was to quantify the potential impact of these practices. Parameterization Plant types In this simulation, three plant types were chosen to represent pasture succession. Pasture grass is represented by Brachiaria spp., weeds by Borreria spp. and early successional trees by Vismia spp. Death Rate A Brachiaria plant has a life span of 3 years. Borreria can be killed by fire or overtopped by woody species. Vismia has a life span of at least 14 years. Reproduction Brachiaria can reproduce through tillers that occupy each neighboring grid cell. Borreria seeds are disseminated close to the parent plant with a much higher probability than further away. Vismia can send out root sprouts to each neighboring grid cell after it is 12 months old. A Vismia seed can only germinate in a grid containing Borreria that is one year or older. Grazing At a high grazing level 70% of all edible Brachiaria plants are grazed every 2 months. The presence of grazers produces bare patches due to trampling and overgrazing. Simulated succession under high grazing Impact of management Standing Biomass and NPP After 1 year, Brachiaria is dominant. After 2 years, Brachiaria with invading Borreria. Results In this simulation study, low grazing resulted in rapid decline of NPP of Brachiaria, rapid weed invasion resulting in an acceleration of dominance by Vismia. This led to higher standing biomass at the end of 14 years relative to other management scenarios. With high grazing, weeds are never as dominant as in low grazing systems because more Brachiaria remain vigorous. With frequent burning (2, 3 and 4 years) and high grazing, Brachiaria covers 50% of the pasture even after 8 years. But by 12 years, there is only a small amount of grass and weed coverage in any of the systems. The model predicts that frequent burning will result in more bare patches even in the later stages of succession. After 6 years, Borreria and Brachiaria with Vismia invading. After 14 years, Vismia is dominant. Percent coverage of Brachiaria, Borreria, Vismia and patches with different management practices. Yellow is Brachiaria, green is Borreria, blue is Vismia . References Deursen, W. van and G. Heil. 1993. Analysis of heathland dynamics using a spatial distributed GIS model. Scripta Geobot. 21:17-27. Karssenberg, D. 1995. PCRaster manual version2. Utrecht University, The Netherlands. 355 pp. Rupp, T.S., Starfield, A.M. and F.S. Chapin. 2000. A frame-based spatially explicit model of subartic vegetation response to climatic change: comparison with a point model. Landscape Ecology 15:383-4000. Scanlan, J.C. 1994. State and transition models for rangelands. 5. The use of state and transition models for predicting vegetation change in rangelands. Tropical Grasslands. 28: 229-240. Abandoned pasture after 2 years, containing Rolandra fruticosa, Brachiaria humidicola, B. brizantha, and an unfamiliar broadleaf species Abandoned pastrue after 5 years, containing Vismia , Borreria verticillata, Brachiaria humidicola, B. brizantha, Solanum spp., Lantana camara